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標題: | 新穎電子施體/受體共軛高分子之合成、形態及薄膜電晶體與太陽能電池元件應用 Syntheses, Morphology, and Properties of New Donor-Acceptor Conjugated Polymers for Field Effect Transistor and Solar Cell Applications |
作者: | Jung-Hsun Tsai 蔡榮訓 |
指導教授: | 陳文章(Wen-Chang Chen) |
關鍵字: | 共軛高分子,電子施體/受體,有機太陽能電池,薄膜電晶體, conjugated polymer,donor-acceptor,organic solar cell,field effect transistor, |
出版年 : | 2010 |
學位: | 博士 |
摘要: | 電子施體/受體共軛高分子由於可藉由分子內電荷轉移效應調控不同光電性質,使其能廣泛應用於薄膜電晶體與光伏打電池而備受囑目。為了達到良好的元件表現效率,我們必需設計與合成具有高電荷傳輸效率、規整與可調控的分子結構和適當的分子能階的共軛高分子結構。因此,此博士論文目標在於引入高推電子性與高共平面性的基團做為電子施體,以期能促進分子間的排列與電荷傳導。本論文中架構出的新穎電子施體/受體系統,包含: (1) 吲哚咔唑(IC)系列,(2) 聚茚噻吩(TPT)系列,(3) 二維排列噻吩(4T)系列等共軛高子,針對其合成、光電性質鑑定與元件應用系統性的探討。除此之外,我們藉由添加軟鏈-硬桿-軟鍵三嵌共聚物為界面活性劑,進而提升以聚(3-己烷基噻吩) /碳六十衍生物做為主動層的元件效率。茲詳細介紹如下:
1. 吲哚咔唑電子施體/受體交替共聚高分子之合成、性質鑑定及其薄膜電晶體與有機太陽能電池應用 (第二章) 合成不同接枝的吲哚咔唑(28IC與39IC)與四種不同強度電子受體 (TP12、TPO、BT與QO) 所形成的六種電子施體/受體共軛高分子,並探討其光電性質與元件應用。其高分子能隙 (eV) 的順序為P28IC-TPO (1.58) < P39IC-TP12 (1.79) < P28IC-TP12 (1.84) < P28IC-BT (2.09) < P28IC-QO (2.31) < P39IC-QO (2.34)。而在電晶體量測可以觀察到其電動遷移率約為1.66x10-5 至4x10-4 cm2/Vs–1之間,電流開關比約為40至46900之間。另一方面,將此共軛高分子材料混摻碳六十或碳七十衍生物做為有機太陽能電池主動層,其光電轉換效率於0.14%至1.40%之間。對於P28IC-QO,因為其具有最佳的HOMO/LUMO能階位置、高分子量、高電荷傳遞速率、最大螢光發光淬息(PL quenching)與高開路電壓使其達到最大光電轉換效率1.4%。實驗結果反映出不同接枝位置的吲哚咔唑與不同電子施體強度之間的電荷轉移效應會嚴重影響其光電性質,進而達到不同的元件表現。 2. 聚茚噻吩電子施體/受體隨機共聚高分子之合成、性質鑑定及其光電元件應用 (第三章) 合成以聚茚噻吩(TPT)為電子施體,搭配不同比例電子受體強度的DTQ與DPP單元所形成的隨機共聚高分子(PTPTDTQ0.55、PTPTDTQ0.34DPP0.14與PTPTDTQ0.26DPP0.34)。其高分子能隙為1.74(PTPTDTQ0.55)、1.56(PTPTDTQ0.34DPP0.14)與1.48 eV (PTPTDTQ0.26DPP0.34)。而PTPTDTQ0.55、PTPTDTQ0.34DPP0.14與PTPTDTQ0.26DPP0.34之電洞傳遞速率(cm2 V–1 s–1)/電流開關比分別為2.2 × 10–3/4.0 × 104、2.4 × 10–3/4.0 × 104與4.7 × 10–3/5.3× 104。由以上結果可知,當添加較多比例的強電子受體-DPP單元時,會造成較低的電子能隙與高電洞傳遞速率,這也顯示出在TPT電子施體與DPP電子受體間的高電荷轉移效應不但能降低能隙亦能幫助電荷傳輸。另一方面,PTPTDTQ0.55混摻碳七十衍生物做為有機太陽能電池主動層,達到最佳光電轉換效率3.71% 。除此之外,我們也首次將聚茚噻吩系統引入光偵測器元件 (photodetector)的應用。相同地,以PTPTDTQ0.26DPP0.34混摻碳七十衍生物做為光偵測元件吸光層,發現在700 nm吸收波段具有超過32%外部量子轉換效率(-3V偏壓下)與高元件應答速度。綜合上述,聚茚噻吩(TPT)電子施體/受體系統在多樣光電元件應用上皆具有良好的表現。 3. 二維排列噻吩電子施體/受體交替共聚高分子之合成、性質鑑定及其薄膜電晶體與有機太陽能電池應用 (第四章) 合成以二維排列噻吩(4T)搭配四種不同強度電子受體 (BT、DTBT、DTQ與DPP)所形成的四種電子施體/受體共軛高分子。其高分子能隙(eV)的順序為P4TDPP (1.29) < P4TDTBT (1.60) < P4TDTQ (1.83) < P4TBT (1.88)。而其電動遷移率約為10-1 至10-4 cm2/Vs–1之間。有趣的是,P4TDTBT與P4TDPP由於LUMO能階較低,適合電子注入,使其具有雙載子(ambipolar)電晶體傳輸特性。對於P4TDPP而言,其p型電晶體之電洞傳遞速率(cm2 V–1 s–1)/電流開關比為1.15 × 10–1/2.49 × 104; 同時,n型電晶體之電子傳遞速率(cm2 V–1 s–1)/電流開關比為 3.08× 10–3/7.34 × 102。P4TDPP在電晶體上良好的表現是因為其規整的分子鏈排列,可進而由DSC與XRD的探討得到佐證。另一方面,以此共軛高分子系統混摻碳七十衍生物(1:3, w/w)做為有機太陽能電池主動層,可達到光電轉換效率在1.28至1.67%之間。我們更深入的探討P4TDPP混摻不同比例碳七十衍生物(1:1至1:4)做為主動層,當混摻比例為1:2時,最能達到電子/電洞傳輸平衡,進而獲得最佳光電轉換效率2.43%。綜合上述,二維排列噻吩(4T)電子施體/受體系統具有增進分子結構排列的特性,因此在光電元件的應用上深具潛力。 4. 聚(3-己烷基噻吩)/碳六十衍生物太陽能電池元件效率最佳化 : 添加含有聚(3-己烷基噻吩)與聚(4-乙烯基苯胺)鏈段的三嵌共聚物為界面活性劑 (第五章) 我們導入軟鏈-硬桿-軟鏈三嵌共聚物(PTPA-P3HT-PTPA)於聚(3-己烷基噻吩)/碳六十衍生物(P3HT/PCBM, 1:1)太陽能電池元件做為調控混摻結構的界面活性劑(相容劑)。當添加0%-1%和2%-5%時,其顯示為纖維狀的表面微結構,然而只有在添加1.5%時,會發現形成比纖維狀尺度更小的圓球狀的結構。而此更小的圓球表面聚集可以增加電子/電洞對分離的表面積。另一方面,也由於添加了具電子施體效應的PTPA-P3HT-PTPA,具有提升混摻系統中電洞傳遞速率的效應,進而讓電子/電洞傳輸更為平衡。綜合以上二個效應,當添加1.5%的相容劑時,系統能達到最佳光電轉化效率4.4%。比較不添加相容劑的系統 (3.9%),效率有明顯的提升。除此之外,添加相容劑亦能改善太陽能電池元件的空氣穩定性與熱穩定性。而由DSC的研究亦指出,相容劑的軟鏈段會選擇性的與PCBM互溶,進而抑制在加熱過程中,PCBM會形成的結晶聚集。綜合上述,此三嵌共聚物在P3HT/PCBM混摻系統中具優異的相容效應,不但提升了光電轉換效率亦增進了元件穩定性。 Donor-acceptor (D-A) copolymers have attracted significant scientific interest recently as their electronic and optoelectronic properties can be manipulated through intramolecular charge transfer (ICT). Such polymers may have potential applications in various organic electronic devices, especially for field effect transistors and polymer solar cells. In order to obtain high-performance of these devices, it is essential to design and synthesize conjugated polymers with desirable properties, such as high carrier mobility, ordered and tunable morphology, and appropriate molecular energy levels. Utilizing highly electron-donating and coplanar aromatic fused-rings as a building block into D-A based system could probably meet above-methioned requirements, contructing ordered molecular packing and high charge transporting of conjugated polymers. Herein we summarized the systematic studies on the syntheses, optoelectronic properties, morphology, and their device characterizations of the three D-A conjugated systems embedded with coplanar donors, including: (1) indolocarbazole-based, (2) thiophene-phenylene-thiophene (TPT)-based, (3) two-dimensional thiophene (2D)-based alternating or random copolymers. In addition, the final subject is to introduce P3HT-based coil-rod-coil copolymer into bulk-heterojunction solar cell as a surfactant, leading to the improved efficiency and device stability. The details of each topic are summarized as below: 1. Synthesis of New Indolocarbazole-Acceptor Alternating Conjugated Copolymers and Their Applications to Thin Film Transistors and Photovoltaic Cells (Chapter 2): we report the synthesis, properties, and optoelectronic device characteristics of six new indolocarbazole-acceptor conjugated copolymers prepared by Suzuki coupling reaction. Two different linkages of indolocarbazole (28IC and 39IC) and four acceptors of 2,3-didodecylthieno[3,4-b]pyrazine (TP12), 2,3-bis(4-(2-ethylhexyloxy)phenyl)thieno[3,4-b]pyrazine (TPO), 2,1,3-benzothiadiazole (BT), and 2,3-bis(4-(2-ethylhexyloxy)phenyl)quinoxaline (QO) were used to explore the effects of acceptor structure, linkage, and side group on the electronic and optoelectronic properties. The hole mobility and on-off ratios of the studied copolymers were in the range of 1.66×10-5 ~ 4×10-4 cm2 V–1 s–1 and 40~46900, respectively. It basically depended on the degree of intromolecular charge transfer (ICT) between indolocarbazole and acceptor as well as the HOMO level. The power conversion efficiency (PCE) of the indolocarbazole-acceptor polymer/PC61BM or PC71BM based photovoltaic cells were in the range of 0.14-1.40% under the illumination of AM 1.5G (100 mW/cm2). P28IC-QO showed the best PCE among the studied copolymers because of its suitable HOMO/LUMO energy level, high molecular weight, good hole mobility, efficient PL quenching, and large Voc. 2. New Thiophene-Phenylene-Thiophene Acceptor Random Conjugated Copolymers for Optoelectronic Applications (Chapter 3): New low band-gap thiophene-phenylene-thiophene (TPT)-based donor-acceptor-donor random copolymers were synthesized for optoelectronic device applications by a palladium-catalyzed Stille coupling reaction under microwave heating. The acceptors included 2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-bis[5’-bromo-dithien-2-yl-quinoxa- lines] (DTQ) and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethyl-hexyl)pyrrolo[3,4-c] -pyrrole-1,4-dione (DPP). The prepared the random copolymers were named as PTPTDTQ0.55, PTPTDTQ0.34DPP0.14, and PTPTDTQ0.26DPP0.34 depending on the copolymer ratio and their corresponding Egopt (eV) were 1.74, 1.56, and 1.48 eV, respectively. The hole mobility obtained from the field effect transistor devices prepared from PTPTDTQ0.55, PTPTDTQ0.34DPP0.14, and PTPTDTQ0.26DPP0.34 were 2.2×10–3, 2.4×10–3, and 4.7×10–3 cm2 V–1s–1, respectively, with the on-off ratios of 4.0×104, 4.0×104, and 5.3×104. It suggested that the significant ICT effect between the TPT and acceptor led to the band gap reduction and hole mobility enhancement. Polymer solar cells of these TPT-based copolymers blended PC71BM exhibited power conversion efficiencies (PCEs) as high as 3.71 %. Besides, the near-infrared photodetector device prepared from PTPTDTQ0.26DPP0.34 showed a high external quantum efficiency exceeding 32% at 700 nm (under –3V bias) and fast-speed response. The present study suggests that the prepared TPT-based donor-acceptor random copolymers exhibited promising and versatile applications on optoelectronic devices. 3. New Two-Dimensional Thiophene-Acceptor Conjugated Copolymers for Field Effect Transistor and Photovoltaic Cell Applications (Chapter 4): we report the synthesis, properties and optoelectronic device applications of two-dimensional (2D) like conjugated copolymers, P4TBT, P4TDTBT, P4TDTQ, and P4TDPP, consisted of 2’,5’’-bis(trimethystannyl)-5,5’’’-di-(2-ethylhexyl) -[2,3’;5’,2’’;4’’,2’’’]quarterthiophene (4T) with the following four acceptors of BT, 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT), DTQ, and DPP. The 2D-like conjugated copolymers exhibited high hole mobilities in the range of 10-1 ~ 10-4 cm2 V–1s–1. Moreover, the FET electron mobilities were observed for P4TDTBT and P4TDPP, due to their relatively low-lying LUMO level suitable for electron injection. In particular, P4TDPP showed the ambipolar characteristics with the hole and electron mobilities of 0.115 cm2V–1s–1 (on/ff ratio : 2.49×104) and 3.08×10-3 cm2V-1s-1 (on/off ratio : 7.34×102), respectively, which was strongly related to its order intermolecular chain packing based on the DSC and XRD studies. The PCE could be reached to 2.43 % of P4TDPP/PC71BM (1:2) based device, due to the balanced hole/electron mobility. The above results indicate that these two-dimensional 4T-acceptor conjugated copolymers could enhance the charge-transport characteristics and are promising materials for organic optoelectronic devices. 4. Enhancement of P3HT/PCBM Photovoltaic Efficiency Using the Surfactant of Triblock Copolymer Containing Poly(3-hexylthiophene) and Poly(4-vinylphenylamine) Segments (Chapter 5): the well-defined coil-rod-coil triblock copolymer, PTPA-P3HT-PTPA, has been served as a surfactant for P3HT/PCBM (1:1) based solar cells. The device performance is enhanced in the presence of the 1.5% PTPA-P3HT-PTPA with optimized devices showing a power conversion efficiency of 4.4%. With the surfactant ratios in the range of 0% to 1% and 2% to 5%, the fiber-like structure could be observed. As the critical ratio of 1.5% is added, the sphere-like nanostructure were obtained resulting to smaller domain size and increasing the interfacial area for charge separation as compared to fibrous structure. On the other hand, the increasing hole mobility with the addition of surfactant maybe due to the donor characteristic of PTPA-P3HT-PTPA, leading to the balanced hole and electron mobility. Hence, the significant enhanced PCE of the 1.5% PTPA-P3HT-PTPA blended system as compared to the pristine P3HT/PCBM system (3.9%) could be attributed to the sphere-like structure formed and much more balanced mobility (μe/μh ~1.7). Additionally, the introduction of PTPA-P3HT-PTPA as a surfactant not only extents the life-time of solar cells but also reduces the PCBM aggregation upon annealing, resulting in better thermal stability. The surfactant effect also could be confirmed by DSC measurement revealing that the selective miscibility of coil segment with PCBM. These results indicate the superior compatibilizing effect for the triblock copolymer in solar cell application. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/47579 |
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